1. Field of the Invention
This invention relates to the testing of materials for fracture toughness, fatigue crack growth resistance and stress corrosion crack growth resistance, and more particularly, to a simple portable hand held loading device which is fully mechanical and which provides direct reading of fracture toughness using the short rod/bar specimen geometry.
2. Description of the Prior Art
Fracture toughness, K.sub.Ic, is an intrinsic material property and is a measure of the energy required to create new surface area in a material. Fracture toughness measurements can be made for a wide range of rock types for example. As a measure of energy comminution, fracture toughness might be used for more sensitive predictions of tunnel boring machines (TBM) performance than are possible with other index measures in current use. Such testing is quite often done with specimens of short rod/bar geometry which include a machine slot where crack growth initiates from the root of the slot. Under test, load is applied to the specimen which tends to increase the machine slot width. Material testing machines conventionally involve arms that fix to parts of the specimen and which, when energized, move apart to widen the specimen slot, thus effecting fracture of the specimen.
Fairly sophisticated machines have been developed for loading of a slotted specimen and for measuring the fracture toughness, fatigue crack growth resistance, or stress corrosion crack growth resistance of that specimen. U.S. Pat. No. 4,075,884 issuing Feb. 28, 1978, is illustrative of one machine of the hydraulic type involving a thin pressure bag installed within the slotted portion of the specimen and expanded by application of hydraulic fluid to produce a desired specimen loading. By measuring the hydraulic pressure at fracture, a determination of the load applied to the specimen is ascertained. Such materials testing machine is complicated and has all the attendant problems of dealing with a liquid under pressure, such as loss of seal and the requirement to transmit the hydraulic fluid to and from the pressure bag.
U.S. Pat. No. 4,198,870 issuing Apr. 22, 1980 describes a second type of sophisticated machine wherein a pair of fixed outer arms at their upper ends function as a pivot for a rigid center arm with each arm arranged to mount a grip coupled below the pivot via opposed grooves to a short rod, short bar or other fracture test specimen. The machine is further characterized by a motor and screw means turning in a threaded collar for pivoting the center arm relative to the side arms. Spring gauge means secured to the middle arm senses flexure and indicates the load forces required to achieve fracture. While this machine operates satisfactorily to adequately measure the applied force, a motor is required to provide the applied force, the stress on the test specimen is sensed separately from the force application means, and the mechanism does not lend itself to portability and hand held use.
It is, therefore, a primary object of the present invention to provide an improved hand held, direct reading, fracture loading device for short rod/bar specimens which employs a mechanical force gauge to measure directly the maximum load reading required by the specimen geometry and in which the load applied to the specimen is applied through the force gauge itself.
It is a further object of the present invention to provide such a fracture toughness measuring device which is actuated solely by hand turning of a knob borne by the unit and which eliminates complex electronic or hydraulic components characterizing prior machines.